The invention relates to an arrangement in a tunable magnetron comprising a sleeve-shaped body which by means of two bearings is rotatably journalled on a stationary column or pillar and which at one end supports a tuning body projecting into the resonance cavities of the magnetron. An inner bearing part of a bearing has a fixed position relative to the column and an outer bearing part of a bearing has a fixed position relative to the sleeve-shaped body, while the distance between the bearings is determined by distance means.
A magnetron of this general construction is described in Swedish Pat. No. 191,373 (corresponding to U.S. Pat. No. 3,343,031), for example. The tuning body here has portions of different electric conductivity, formed, for example, by means of circumferentially distributed teeth or apertures in the body, and projects through a gap made in the rear part of the anode plates defining the resonance cavities. In order to achieve a high efficiency the gap is made very small, as large gaps between the tuning body and the anode plates will reduce the efficiency.
Small gaps will result in high requirements on the bearing, in particular as regards freedom from play. Due to the small dimensions of the gaps there is already a very small play, and consequent inclination of the sleeve-shaped body will result in an appreciable influence on the electric HF signal generated by the magnetron, in particular the frequency of the signal. Play in the bearings can furthermore result in vibrations so that the operating life of bearings and thereby of the whole magnetron will be reduced. Very high requirements are therefore imposed upon the bearing for both electrical and mechanical reasons.
To avoid play in the bearings it is important that both bearings are loaded, or in other words that they are biased. The biasing force can be achieved in different ways depending upon how the contact lines through the contact points in the bearings are oriented. In principle the contact lines can be parallel or intersecting. The latter lines can intersect each other either between or beyond the bearings. These different types of biasing forces are often called: "tandem", "face--to-face" and "back-to-back". The parallelism and symmetry can be more or less exact, dependent on practical circumstances.
Besides freedom from play it is of great importance that the bearings are not too heavily loaded. The biasing involves, as a rule, a certain increase of the friction in the bearing and this friction must be kept low and accurately limited.
These requirements should be fulfilled even in several operating conditions, i.e. involving operation of the bearings in a vacuum and under varying temperature conditions. The temperature will vary from the surrounding temperature at the start to varying high temperatures during operation, depending upon frequent variations of the electric power applied to the magnetron and variations of the microwave power delivered by the magnetron. Due to the effective thermal insulation between the different parts in the radial direction of the bearing arrangement there is furthermore, in the steady state, a high temperature gradient in the radial direction. In contrast to this, the temperature gradient in the axial direction is small because both the central column supporting the whole bearing arrangement and the rotatably-journalled sleeve are usually made of materials having good heat conductivity. The bearings must operate without play and with a low friction within the whole temperature range.
In such magnetrons it is usual that the sleeve-shaped body is influenced by a continuous axial magnetic force. By means of this force a biasing of the tandem type can be achieved. It is then important that both bearings are loaded and in such a way that they have the same loading and each take up half the force.
Many solutions of the bearing problem in tunable magnetrons of the above-described kind have been proposed. A bearing arrangement is described in European Pat. No. 0009903 (corresponding to U.S. Pat. No. 4,281,273), for example in which both the inner rings and the outer rings of the bearings are displaceably arranged on a fixed column in the rotating sleeve body. The outer rings of the bearings together with a distance sleeve arranged between them are pressed against a fixed stop on the rotating sleeve. The inner rings are, on the one hand, influenced by a spring pressing the whole assembly of inner rings and intermediate distance elements against a stop on the column and on the other hand, by a spring included in the distance element and pressing the two inner rings away from each other. The stop on the column is furthermore adjustable in the axial direction. This adjustment of the stop on the column is then carried out in such manner that the load is distributed in the desired manner between the bearings. During the adjustment, as well as during thermally-induced variations during operation, the inner rings of the two bearings will be displaced on the column. The adjustment for achieving the desired distribution of the load between the bearings is very critical. If the spring characteristic of the springs should vary with time adjustment will be erroneous. Another drawback of this arrangement is that the inner and outer bearing rings must have loose tolerances against the column and the sleeve body, respectively, which in itself involves play and can give rise to vibrations.